1. Field of the Invention
The present invention relates to a comparator circuit, a semiconductor device, a battery monitoring system, a charging prohibition method, and a computer-readable medium in which is stored a charging prohibition program. The present invention particularly relates to a comparator circuit that compares a voltage of a positive electrode and a voltage of a negative electrode of a battery cell, a semiconductor device, a battery monitoring system, a charging prohibition method, and a computer-readable medium in which is stored a charging prohibition program.
2. Description of the Related Art
Generally, batteries (specific examples include lithium ion batteries) that are assembled batteries in which plural batteries (battery cells) that are rechargeable are connected in series are used as large-capacity, high-output batteries used to drive motors in hybrid vehicles and electric vehicles. Battery monitoring systems provided with a protection circuit for protecting the battery are known.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 11-155241, there is described a charging current control circuit and an assembled battery charging method that can, when charging an assembled battery in which plural battery cells that are rechargeable are connected in series, fully charge all battery cells even when using battery cells with different characteristics.
In a battery monitoring system like this, in a case where the voltage of a battery cell is extremely small, there are cases where irreversible deterioration of the battery cell occurs. For example, in a lithium ion battery cell, when the battery voltage becomes equal to or less than 0.6 V, the copper of the copper foil that is the coated base material of the negative electrode ionizes, dissolves into the electrolyte, and precipitates everywhere inside the battery cell. Because of this, the function of the positive electrode deteriorates, the container (iron) dissolves, holes form, and the electrolyte leaks. Moreover, in a case where the electrolyte has leaked, charging current flows through the electrolyte, so sometimes the printed board to which the electrolyte has adhered gives off heat, gives off smoke, and moreover ignites.
In order to respond to the above situation, a charging prohibition voltage is determined, and generally in a case where, even in one of the battery cells included in the assembled battery, the battery voltage thereof has become equal to or less than the charging prohibition voltage, a predetermined processing such as prohibiting charging is performed.
For that reason, the protection circuit of the battery monitoring system is configured to include a voltage detection circuit for detecting whether or not the battery voltage of a battery cell has become equal to or less than the charging prohibition voltage. FIG. 13 shows a specific example of a conventional voltage detection circuit 2000. The voltage detection circuit 2000 is disposed for each battery cell in a plurality of battery cells connected in series (see FIG. 2). As shown in FIG. 13, the conventional voltage detection circuit 2000 is configured by a comparator circuit that compares a voltage obtained by dividing the battery voltage of each battery cell (the difference between the voltage of the high potential side and the voltage of the low potential side) by the resistance with a reference voltage generated by a reference voltage generating circuit and outputs the comparison result.
However, the above technology cannot detect the battery voltage in a case where the battery voltage of the battery cell is low.
The reference voltage generating circuit and the comparator circuit configuring the voltage detection circuit 2000 shown in FIG. 13 are generally configured by CMOS (complementary metal-oxide semiconductor) integrated circuits, so the lowest operational power supply voltage is constrained by the threshold value Vt of the MOS transistors. The power for the reference voltage generating circuit and the comparator circuit is supplied from each battery cell, so when the battery voltage of the battery cell becomes an extremely low voltage, such as equal to or less than 1 V, for example, the power supply voltage of the reference voltage generating circuit and the comparator circuit ends up becoming equal to or less than 1 V.
The aforementioned charging prohibition voltage differs depending on the battery cell, but generally a voltage of about 0.7 V is used. Further, in recent battery cells, there is a desire to make the charging prohibition voltage about 0.1 V to about 0.2 V.
However, in the voltage detection circuit 2000, if the battery voltage of the battery cell is lower than the lowest operational power supply voltage, there are cases where the reference voltage generating circuit and the comparator circuit do not operate. Consequently, the voltage detection circuit 2000 can only, being subject to the constraint of the lowest operational power supply voltage, detect a battery voltage that exceeds the lowest operating power supply voltage. In a case where the battery voltage of the battery cell is an extremely low voltage, such as equal to or less than 1 V, the voltage detection circuit 2000 cannot detect the charging prohibition voltage because it cannot detect the battery voltage.
Particularly in a case where the battery voltage of the battery cell has decreased abruptly, the battery voltage that has become low cannot be detected, so a situation such as described above ends up occurring because charging cannot be prohibited and the battery cell whose battery voltage is equal to or less than the charging prohibition voltage ends up being charged.